The invention relates to a method for the preparation of novel photopolymers based on prepolymer-based polyurethane compositions which are suitable for the production of holographic media, in particular for the visual display of images.
Photopolymers are materials which can be exposed by means of the superposition of two coherent light sources, resulting in a three-dimensional structure in the photopolymers, which generally can be written in the material by a regional change of the refractive index. Such structures are referred to as holograms. They may also be described as diffractive optical elements. The optical functions developed by such a hologram depend on the specific exposure to light.
Polymer formulations for the production of holographic media are described, inter alia, in the non-prior-published patent applications EP 08017279.4, EP 08017277.8, EP 08017273.7, EP 08017275.2. EP 08017277.8 and EP 08017273.7 describe polyether-based and prepolymer-based polyurethane compositions which are in general suitable for the production of holographic media. Polyurethane formulations which comprise specific acrylates and are suitable for the recording of holograms were described in EP 08017275.2. Furthermore, a typical film structure and the application of various polyurethane formulations as a photopolymer in the film composite was described for the first time in EP 08017279.4.
The production of such holographic media as a glass or film composite has been effected in the prior art to date in individual process steps which are not suitable for production in a manner similar to manufacture and to meet requirements.
For the production of such film composites in a manner similar to manufacture, film coating units which are to be understood as a continuously operated roll-to-roll process are used. In general, film coating is understood as meaning the application of a liquid substance to a moving film material. The coating processes differ, inter alia, according to the properties of the layer to be applied (e.g. viscosity, surface tension, solids concentration, etc.), the target layer thickness to be applied or the manufacturing speed.
The film coating units to be used for manufacture typically consist of:                Unwinding from a roll, which ensures the feeding of the support material (film substrate).        Pretreatment of the coating material: accessory units and peripheral devices, such as, for example dissolvers and kneading units, weighing systems, various pump systems, automatic solids feeds, consumption measuring systems, filter units, devolatilization devices and mixing systems are used for the pretreatment.        Pretreatment of the support material: in the coating industry, web cleaning systems for cleaning the support material, ionization units known to the person skilled in the art and intended for homogenizing the electrical charges present on the support material, plasma and corona treatments are used as pretreatment units. Furthermore, web control systems which are supported by devices such as, for example, web edge control and web storage systems are used.        Application system for applying a coating material: various knife coater and spreading systems, feed hoppers with slot die and roller application systems are used in the coating industry as application systems, as well as systems such as, for example, curtain and cascade processes for multilayer application.        
Coating devices which can be used, such as, for example, curtain or cascade coaters or slot nozzles, are described in earlier publications, for example (P. M. Schweizer, “Vordosierte Beschichtungsverfahren: Vorzüge and Anwendungen [Premetered coating methods: advantages and applications]”, Coating, December 1998 edition, pages 462-465): thus, Schweizer describes premetered coating methods which were developed as long ago as the fifties, inter alia in the photographic industry. In these methods, liquid is distributed transversely to the support material by homogeneous pressure distribution within a nozzle. These methods are used for single-layer as well as for multilayer products (slot nozzle ≦3, cascade and curtain methods with >10). In the slot and cascade methods, the nozzle is separated from the support material only by a very narrow gap. In the curtain method, the nozzle is present several centimeters away from the support material so that the liquid distribution is substantially decoupled from the coating process.
Further, all customary roll application systems, which differ specifically through the arrangement, designs and number of rolls, are used. The coating material to be applied can be stored either in a tank or in the gap between two horizontally arranged rolls for metering. In all cases, the rotation speed of the wetted rolls, the nature (engraving, smooth rolls) of the rolls, viscosity and surface tension influence the layer thickness to be applied, in relation to the substrate speed. In addition, with increasing substrate speeds, further rolls are used which result in further extension of the coating material through differently adapted rotation speeds of the rolls and a defined nip between the two rolls. The last roll of the system is a very small distance away from the support material, and the coating material is therefore pressed onto the support material either in countercurrent or in cocurrent operation. Roll speed, nip, support material speed and the viscosity are decisive factors in establishing the desired wet layer thicknesses.
A 5- or 6-roll application system is described, for example, by Maschinenfabrik Max Kroenert GmbH & Co. KG (M. Schmalz, N. Hansen, W. Neumann, “5-oder 6-Walzen Beschichtungsverfahren [5- or 6-Roll coating method]”, Coating, October 2006 edition, pages 410-413). In this publication, the authors compare the advantages and disadvantages of a 5-roll to a 6-roll application system on the basis of a solvent-free silicone coating with thermally curable or UV-curable silicones.                Dryer units: inter alia, hot air dryers with prethermostated air, IR lamps or UV devices are used for drying the coating material on the support material. Chill rolls, expander rolls, laminating units, calender rolls, cutting devices (such as, for example, edge cutters) are used for the aftertreatment of the coated support materials. In addition, said processes can also be used for solvent-containing systems and are therefore equipped as an explosion-proof unit.        Winding device for winding the coated product or support film.        Said process steps are supplemented in the coating industry additionally by systems for quality assurance, such as, for example, layer thickness measurements, optical surface monitoring systems, atmospheric humidity or solvent concentration measurements and residual product moisture measurements.        
It was an object of the present invention to provide an industrial process by means of which holographic photopolymer films can be produced in a continuous production procedure starting from prepolymer-based polyurethane formulations. The product is to be a film composite comprising cast film, photopolymer and an outer film.
This object was achieved by the specific combination of different process steps in which positive metering pumps known to the person skilled in the art, vacuum devolatilizers, plate filters, static mixers, slot nozzles or various knife coating systems, single-roll unwinders, air dryers, dry lamination device and a single-roll winding device are used. In particular, the coating device, such as, for example, slot nozzles and knife coating systems, are suitable for the application of liquid coating materials, especially in combination with photopolymer formulations, preferably prepolymer-based polyurethane formulations of the following compositions, to moving support materials and are distinguished by high accuracy in the application layer thickness.